1. Field of the Invention
The present invention relates to a high frequency induction heater built in an injection mold for applying a local heat to the plastic, and more specifically, to a high frequency induction heater formed on a side of a stamper by micro electromechanical system (MEMS) technologies.
2. Description of the Prior Art
The injection compression molding technology has become mature in recent years. The injection compression molding technology combines the injection molding technology with the compression molding technology. The injection compression molding technology reduces the injection pressure required when filling the plastic into the cavity. In addition, since the pressure of the melting plastic in the cavity is equally distributed, thus a sink head or a warp problem is prevented. Therefore, the shrinkage of the product is well controlled, in light or the above-mentioned advantages, the injection compression molding technology is normally employed in fabricating optical precision moldings or compact discs. For example, if the compact disc is fabricated by conventional injection molding technology, the plastic cannot be filled completely, which is known as short shot. Thus, thin moldings having large areas, such as compact discs, cannot be formed by conventional injection molding technology. At present, the compact discs are fabricated by injection compression molding technology combining with hot runner design. Since the temperature of the plastic in the sprue is relatively higher, the short shot problem is therefore avoided. U.S. Pat. No. 6,164,952 discloses a method or fabricating DVD discs using injection compression molding technology, in this patent, an inclined angle design is adopted in the cavity for improving the fluidity of the plastic. It is possible to fabricate thin moldings having large areas (diameter: 120 mm; thickness: 0.6 mm) by injection compression molding technology. However, if thinner moldings having larger areas and being coplanar (inclined angle design is not allowed) are desired, or die pattern of die stamper is more complicated (such as the molding includes via holes), and the following problem may occur: If a single sprue method is employed, the plastic cannot be completely filled into the cavity.
2. If a multiple sprue method is employed, and the temperature distribution of the molding is not equal, then the molding may have warps after being cooling.
3. The plastic flow is obstructed and split so that a seam line will generate after the plastic flow converges.
4. Since the molding has large area and thin thickness, if the fluidity of the plastic is not good, the pattern of the microstructure in the stamper will be ruined by the applied pressure.
Generally speaking, 3D micro moldings require precise micro molding injection technology. At present, one of the methods to fabricate 3D micro moldings is carried out by a micro injection machine. The micro injection machine is one of the methods to fabricate complicated and micro plastics, ceramics, and metal parts. Technologically, the injection molding technology is the first choice for fabricating 3D products with a complicated shape. Basically, the micro injection molding technologies are simply classified into 3 types: microstructure injection molding technology, micrometer-level injection molding technology, and micro injection molding technology. All of the three technologies have to overcome the problems such as micro injection machine design, micro mold manufacturing, micro mold flow analysis, micro injection process monitoring, etc. For example, the requirements for the processes of the micro injection machine are listed as follows:
1. An injection machine under 20 tons or a micro injection machine is required.
2. Short detention time is necessary for avoiding the degradation of the plastic.
3. Long injection stroke: the diameter of the screw must be as small as possible (generally the diameter of the screw of the micro injection machine is 4 mm).
4. A long and thin plunger is required.
5. High shear stress is required to lower the viscosity of the plastic.
6. High injection pressure filling is required due to a high flow length/wall thickness ratio and micro channel.
One of the largest shortcomings for the micro injection molding technology is that a precise micro injection machine is required. In addition, the design and manufacturing of the micro injection mold is not standardized yet, thus the number of the molding products cannot reach a mass amount during one single process.
In Japan Patent JP2000-218356, an external heater with sensors is employed to detect the temperature of the movable mold-half and the stationary mold-half and to heat the movable mold-half and the stationary mold-half when the mold is open. Since the fluidity of the melting metal is improved, the metal moldings having complicated structure and large areas can be formed. However, if this method is employed to form plastic moldings having complicated structure and large areas, the following problems may occur:                1. Since the mold is heated when it is open, it is easy for the mold to have an unequal temperature distribution.        2. This method is employed to inject metal material, thus the temperature is too high for plastic materials.        3. This method heats the mold entirely, thus the mold cannot be heated locally according to this method.        4. The mold is heated only when the mold is open, thus the mold temperature is controlled by prediction.        
In light of the above-mentioned problems, the present invention forms a high frequency induction heater on a side of the stamper by MEMS technology. The high frequency induction heater provides two main functions. First, the high frequency induction heater applies a local heat to sections of the plastic having a thin thickness or sections having a large difference of cross sectional areas so that the plastic remains fluid. Second, when the temperature of the plastic molding is not equally distributed, the high frequency induction heater can adjust the overall temperature so that the temperature difference is reduced.
In MEMS industries, since the precise injection molding technology is mature and the cost of plastic material is cheap, polymers such as plastics are used to fabricate housings or covers. For a long time, optical wafers, bio wafers, and communication passive devices are fabricated by LIGA technology and hot embossing molding. Hans-Dieter Bauer et al. produces optical waveguide devices by LIGA technology and hot embossing molding. Since the refractive index is one of the key factors that influence the transmission of light, the precision and accuracy of the size and relative position of the optical waveguide device is important. Generally speaking, the hot embossing molding can form the optical waveguide device. However, the hot embossing molding technology cannot apply an equal pressure so that the moldings having complicated structure and large areas are not easy to be formed. In addition, the production rate is not outstanding, and the microstructure of the hot embossing mold is easy to be broken when being pressurized.
The present invention forms a high frequency induction heater on a side of the stamper such that moldings having a microstructure or large areas, such as optical wafers, bio wafers, and communication wafers, can be well defined. In combination with a substrate having MEMS devices or ICs thereon, a wafer-level package can be made. In such case, the cost of individual package will be enormously reduced.